Field of the Invention
The present invention relates to an automatic focus adjustment apparatus and an automatic focus adjustment method of a camera system, particularly, a technique for driving a lens.
Description of the Related Art
There are two major control methods for the automatic focus adjustment. One is a one-shot mode and the other is an artificial intelligence (AI) servo mode. The one-shot mode (i.e., a single autofocus (AF) mode) is used for capturing an image of a stationary object. The autofocusing is started when a shutter button is half-pressed or when other operations are performed, and once the object comes in focus, the focus is locked. The AI servo mode (i.e., a continuous AF mode) is used for capturing an image of a moving object. Unless the shutter is completely turned off, focus detection and lens driving are continuously performed at intervals.
Conventionally, regarding the drive of a focusing ring (i.e., a focus operation member that operates the drive of the lens to bring the object into focus) of a lens, there has been the following trade-off relationship between stopping accuracy and driving time: if priority is given to the stopping accuracy, longer time is required whereas if priority is given to the driving time, stopping accuracy is reduced. In the present circumstances, however, the user-settable settings of cameras are limited to the driving amount and the driving speed of the focusing ring. If the one-shot mode is employed for the automatic focus adjustment, since a user presses the shutter after confirming the in-focus state, higher priority should be given to the stopping accuracy so long as the shutter is not pressed by the user. If the AI servo mode is employed for the automatic focus adjustment, higher priority should be given to the driving time than the stopping accuracy if the drive of the focusing ring is not finished within a predetermined time during the tracking of a moving object.
Japanese Patent Application Laid-Open No. 2009-128611 discusses a camera employing the above-described technique. When the camera performs moving object prediction control, if a focus adjustment lens fails to reach a drive target position and, accordingly, the lens driving is not stopped within a lens driving initial period, a predetermined extension period is set by a lens driving period extension unit and a moving object prediction recalculation unit. Then, during the driving period extension period, the drive control of the focus adjustment lens is performed based on the target position calculated by the moving object prediction recalculation unit.
However, since the user-settable settings of the camera are limited to the driving amount and the driving speed of the focusing ring, the driving end time of the camera is unpredictable. The one-shot mode and the AI servo mode therefore may not function as required. FIG. 9 illustrates such a situation in the conventional techniques by using the relationship between the focusing ring driving speed and time. According to the conventional technique discussed in the above-described Japanese Patent Application Laid-Open No. 2009-128611, the number of frames per unit time (i.e., the frame speed) is likely to gradually decrease, resulting in decreased marginal performance.